The human eye can perceive a wider range or gamut of colors than can be presented by devices for generating images such as computer monitors, photographs, and printers. An image represented in digital form which contains colors falling beyond the gamut of the device for presenting the image must be restricted by clipping or by compression or by a combination of the two. For a thorough explanation of the problem, see U.S. Pat. No. 5,416,890 to Beretta, assigned to Xerox Corporation. Also, even if the image is to be presented in a device with as broad a gamut of colors as contained in the original image, such as when the original image is captured on film and is to be presented on film, it is often desirable to adjust the brightness or contrast or saturation or hue. Any of these adjustments can cause some of the colors to be generated to exceed the color gamut of the output device.
For digital storage and manipulation by a computer, images are typically stored in a three color cubic space, such as RGB (red, green, blue) as shown in FIG. 3. A simple compression method for restricting the colors of an image to the gamut of a device is to reduce each of the R, G, and B values by a ratio until all values fall within the gamut of the device. This is unsatisfactory because it does not take advantage of a large portion of the colors available and produces an image of poor color quality. A solution which takes advantage of all colors available is to simply clip any values of R, G, and B which exceed the permissible range. This will cause any pixel which is clipped to change its color.
Beretta explains how the color representation can be transformed from a three color representation to a "hue", "chroma" (or saturation), and "lightness" (or value or brightness) representation. There are many mathematical models which implement such a representation system, including CIELAB, HSV, HLS, and TekHVC. See Beretta and Foley, J., COMPUTER GRAPHICS SECOND EDITION, Addison Wesley, 1993, chapter 13.3. Using any one of these mathematical models, the hue can be held constant while the lightness or chroma are clipped or compressed to remap the colors of each pixel so that they fall within the gamut of the output device. In the prior art, it is also known to clip or compress both the lightness and the chroma to reach a color which is the shortest perceptual displacement from the original color (CIELAB.DELTA.E). See U.S. Pat. No. 5,611,030 to Stokes.
Unfortunately, there is no method of clipping or compressing based on a single model which produces the best result in each case. Depending upon the characteristics of the particular image, one or another model using one or another of the methods of clipping or linear compression or non-linear compression may produce the best result.